[Intel-gfx] [PATCH] drm/i915/gt: Fill all the unused space in the GGTT

[Chris Wilson]

When we allocate space in the GGTT we may have to allocate a larger
region than will be populated by the object to accommodate fencing. Make
sure that this space beyond the end of the buffer points safely into
scratch space, in case the HW tries to access it anyway (e.g. fenced
access to the last tile row).

Reported-by: Imre Deak <imre.deak at intel.com>
References: https://gitlab.freedesktop.org/drm/intel/-/issues/1554
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: Imre Deak <imre.deak at intel.com>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/i915/gt/intel_ggtt.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
index d8944dabed55..ad56059651b8 100644
--- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
@@ -191,10 +191,11 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 				     enum i915_cache_level level,
 				     u32 flags)
 {
+	const gen8_pte_t pte_encode = gen8_ggtt_pte_encode(0, level, 0);
 	struct i915_ggtt *ggtt = i915_vm_to_ggtt(vm);
 	struct sgt_iter sgt_iter;
-	gen8_pte_t __iomem *gtt_entries;
-	const gen8_pte_t pte_encode = gen8_ggtt_pte_encode(0, level, 0);
+	gen8_pte_t __iomem *gte;
+	gen8_pte_t __iomem *end;
 	dma_addr_t addr;
 
 	/*
@@ -202,10 +203,16 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 	 * not to allow the user to override access to a read only page.
 	 */
 
-	gtt_entries = (gen8_pte_t __iomem *)ggtt->gsm;
-	gtt_entries += vma->node.start / I915_GTT_PAGE_SIZE;
+	gte = (gen8_pte_t __iomem *)ggtt->gsm;
+	gte += vma->node.start / I915_GTT_PAGE_SIZE;
+	end = gte + vma->node.size / I915_GTT_PAGE_SIZE;
 	for_each_sgt_daddr(addr, sgt_iter, vma->pages)
-		gen8_set_pte(gtt_entries++, pte_encode | addr);
+		gen8_set_pte(gte++, pte_encode | addr);
+	GEM_BUG_ON(gte > end);
+
+	/* Fill the allocated but "unused" space beyond the end of the buffer */
+	while (gte < end)
+		gen8_set_pte(gte++, vm->scratch[0].encode);
 
 	/*
 	 * We want to flush the TLBs only after we're certain all the PTE
-- 
2.20.1